Method and apparatus for measuring persistence screen characteristics



Feb- 1949- y o. H. SCHADE METHOD AND APPARATUS FOR MEAS 2,460,471 PERSISTENCE v URING SCREENCHARACTERISTICS 2 Sheets-Sheet 1 Filedpec. 7, 1945 Hl scr-IADEh Mm@ A URINC PERSISTENCE SCREEN CHARACTERISTICS Filed Dec. 7, 1945 2 Sheets-Sheet? hs C a, mw mw .su .........J L .I lzm. mmwo. CN .C2 .MW S/wm my aw@ Patented Feb. 1, 1949v UNITED STATES PATENT oFFlcE METHOD AND APPARATUS FOR MEASURINGV PERSISTENCE SCREEN CHARACTERISTICS Otto `Heinrich Schade, West Caldwell, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 7, 1945, Serial No. 633,492

17 Claims. (012315-364) This invention relates to the art of measuring light output characteristics of phosphor-coated screens, particularly the type of screen used in cathode ray oscilloscope tubes. Still more specifically the invention has been developed for use in connection with the cascade type screen 'having long persistence.

In the manufacture f cathode ray tubes it is highly desirable to have some method of comparing and rating the light output characteristics of the screens incorporated therein. This is true of cathode ray screens in general but particularly is it true of those screens having long persistence characteristics due to the presenceof a layer of phosphorescent material. The. present invention provides a practical method ofv measuring the light output characteristics of any luminescent screen which can be excited to produce a light output -due to electron bombardment as well as providing portable apparatus easily assembled and used. The invention has been found particularly useful in testing the luminescent screens of tubes tobe used in radar equipment Where the screenv is required to meet rigid and exacting specifications.

One type of long persistence screen is known as a P7 screen and consists of a primary layer of yellow zinc cadmium sulde applied directly to the bulb face and a secondary layer of blue zinc sulde applied on top of the primary layer. In operation, the secondary layer of zinc sulde is excited to blue iluorescence by electron bombardment. During the bombardment period, the primary layer of zinc cadmium suliide will store up much of this blue light energy. When the electron bombardment ceases, the fluorescence subsides immediately while'the primary layer releases its energy as an intense phosphorescent glow. Thus, the image traced by the electron beam is retained for some time after thesignal has ceased and is the method used in this instance to obtain very long persistence. When the screen is properly ma-de the two layers remain well sep-y arated. However, it often Vhappens during the process of manufacture, that the two layers become more or less mixed resulting in a defective tube. For the most part this yunwanted mixing cannot be detected by a mereV visual examination of the screen. But by practicing the present invention, screen mixing is very easily detected sinceit shows up at once in an analysis of the light output persistence measurements.

The invention provides means for reducing the screen to be tested to a base energy level and means to subject it to a, standard excitation.

Means are also' provided for recording the intensity'of illumination on the screen at any time. This permits the recording of the fluorescent light output as well as the. phosphorescent light output.

The invention also provides means for repeating the excitation at any desired interval so that the rate of accumulationof energy by a phosphorescent layer may be measured.

One of the primary requirements, from the standpoint of tube operation, of a screen of the cascade type, is that it have a high level of per# sistent light output. That is, the phosphorescence must have an absolute brightness level high enough to readily distinguish the signal. This absolute intensity is dependent upon the degree of excitation andthe time after excitation at which it is observed. Therefore, any brightness measurement of persistent light must specify these conditions. If the symbol B is used to indcate the brightness of the persistent light, B-l will indicate the persistent light intensity one second after a specied electron excitation of the ,the cascade type.

screen.

, Just as the excitation affects the persistent light intensity level, so the energy already possessed by the phosphor will affect it. The newly acquired energy will add to that already possessed y to result in a higher phosphorescent level at any time after excitation than would be obtained if the phosphor were excited from zero energy level. It is important, then, in comparing luminescent screens, that the excitation of the phosphor be started at the same energy level aswell as thatY visions for its measurement. `Provision is made for starting the screen at zero energy level and subjecting it to a series of excitations at regular intervals. The ratio of the persistent light intensity at a specied time after the last excitation to the persistent light output at the same time after the rst excitation, is termed the buildup ratio for that particular numbery of .excitaf tions. The buildeup ratio will be designated as 5G. An associated number will be used to designate the number of excitations. If Bel indicates the brightness measured onesecond after the first excitation, B-5 would indicate the brightness one Second after the `iifth excitation. The ratio The invention includes pro- Y.

B-S/B-E can be labelled G-z l which is the buildup ratio for five excitations.

lt is also desirable in a cascade type screen such as the P7 to measure its` eiciency. The prif mary eoiency is the intensity o1 fluorescence under a given electron bombardment. The secondary eiciency is the degree with .which the fluorescent light energy becomes converted into phosphorescent light output. The secondary eiiiciency, only, is of importance in testing the long persistence type screen. The fluorescent light output F is measured and compared with the persistent light output Bei. The ratio F/B-i has a maximum limit and will be expressed symbolically as FR, short for dash ratio. The invention provides means and method for measuring this ratio,

One object of the invention, then, is to provide a method for reducing a luminescent screen 4to a zero energy level.

Another-object is to provide a method of measuring the intensity of illumination on a screen at any given time.

Another object is to provide a method of measuring the rate ol accumulation of eneglby a screen containing phosphorescent components.

Another object is to .provid-e a method, of measuring the level of persistent light output .of a screen containing phosphorescent components.

.Still another object is to provide a method o illumination on a screen at any giventiine.

Another object is to provide apparatus which may be employed in measuring the rrate of accumulation of energy by a luininesoentscreen.

raster 7cm. x 'l om. in size, tracing 20) to 250 horizontal lines in /GO sec.; i. e., the frequency of the vertical deflection is 60 cycles and for the horizontal deflection, 12,000 to 15,096 cycles. I.mmediately before the screen is tested the tube Y is biased beyond cutmo. The multivibrator Zilli vAnother,object .is to provide apparatus .which Y may be employed in measuring the level of persistent light output of a luminescent screen.

Another `object is to provide apparatus which may be Yemployed in .measuring .the ypeair light output attained by a luminescent screen .during excitation by electron bombardment.

Another object is to provide apparatus which may be employed to subject a luminescent screen to a series of cycles-,oi standard excitations.

y'These and other objects will be apparent from a more .detailed description of the invention.

' screen to a predetermined energy level.

Fig. 2 is a detailed example showingone em.- bodiment of apparatus which may beemployed in the measurement procedure.

Fig. ^3 is a representation of .the type of trace obtained on the screen of the cathode ray oscilloscope 'when measuring l-l or BFS.

Fig. 4 is a representation of .the type of trace obtained on the screen .oi the cathode ray oscilloscope-When measuring the flash ratio.

As shown 'in Fig. l, 251i represents a Acathode ray tube having a luminescent screen Z5! to be tested. Normal operating voltage is established by deecting the cathode ray beam to form a is set into operation and supplies a negative pulse voltage which is first converted to a positive pulse voltage in the iirst stage ZlilA of a buffer amplifier and then fed to the grid of the cathoderay tube 25%. By adjusting the pulse voltage Vcontrol lie of the multivibrator vthe magnitude of said voltage may be set to nre the `tube at any predetermined beam. current level.

For example, sir" the persistence measurements to be madeat a beam current of 6o inicroarnps, the control grid voltage may be set at --26 v. and the accelerator grid voltage adjusted so that the beam current is equal to 60 microanips. Then, il the tube is biased to Llv immediatelybefore persistence measurement and the modulating pulse set for +25 -v., the multivibrator will cause .the tube fto Ynre at 60 microamps each time the pulse comes through. Any equivalent operation `that may be necessary due to tube type peculiarities will serve just as well. The desired vvoltage may be obtained from a potentiometer in the output circuit of the multivibrator and may be fread from 1a D. C. meter such as indicated at i. Pulse width control 2M onthe panel of the multivibrator is used to supply a pulse of desired duration Vsuch as 1&0 second. The pulse width harge rate of the .multivibrator by varying the point at -v/hich the multivibrator outs oli.

Standard methods of screen excitation also specify that the above vdescribed pulse voltage be repeated at one second intervals. Proper timing may he obtained utilizing the synchronizing pulse L ied by the cathode ray tube or better still by a inicroswitch operated by a small synchronous motor connected in the line which supplies 60 cycle synchonizing pulses to the multivibrator. The control 28,2 on the multivibrator panel varies a resistor whichV may beused to control the interval of repetition.

A. second portion of the output of the multivibrator is diverted through the second stage 248B of the buffer amplifier where the pulses are changed to positive and considerably amplied. These amplified pulses are then ied into a pulse counter circuit 220 having a control iol' varying't'he number of pulses in each cycle. The principle of operation involved here is that a condenser is charged in a predetermined number of steps corresponding to the desired number of `pulses in each cycle and when the charge becomes Vhigh enough it will operate relay itt which impresses a negative voltage on the Vmultivibrator suicient to bias it off. Usually the control 22 l, which operates a variable resistor, is set to provide anumber of pulses between 3 and 8. The counter circuit also provides. the time'base deflection for the .oscilloscope 23B.

Before the `screen Ei of tube '25.9 can be tested it is important Vthat thescreen be reduced to zero energy level. This is easily and conveniently done by placing before the screen, oodlamp 2li?, shown in Fig, 1A, having a clipon red iilter. Y. The lamp may be a G. E. 150.Watt loodlamp and the filter of the type known as footlight red which transmits energy above 559i) angstrom units. The lamp is turned on and ,placed before the screen fora regulated time which may be set by timer 212. In general, good'screendeactivatlon can be accomplished in from l5 to 30 seconds.

A suitable switch sets the pulsing cycle of the multivibrator in operation and a regulated numberk of pulses is applied to the grid 252 vof tube 250. For the running oi the test, the oodlamp the voltage per stage of the phototube may'be varied in 150 volt steps for Calibrating purposes. The unit also has a plate load selector switch 24! for varying the gain. The adjustment of the plate load is necessary in order to get the trace on the oscilloscope withinV a readable or accurate range.

The readings from which built-up and flash ratio (FR) are calculated, are taken from a long persistence oscilloscope 230 such as a ECP'I, having a P7 screen. It is also possible to use a G. E. recording meter for this purpose. This meter is a recording microammeter listed by the General Electric Company as their No.` 32C68. Both the G. E. meter and the oscilloscope can also be used A at the same time for purposes of comparison.

The output current of the photomultiplier, which is proportional to the Vintensity of illumination of the screen under test, is passed through a suitable load resistor and impresses voltages on the deecting plates of the oscilloscope propor-vr tional to light output. By means of a calibrated scale on the face of the tube, the spot deflection can be read directly in millifoot lamberts or any equivalent light unit. The horizontalr sweep of the oscilloscope is synchronized with the pulses of the multivibrator. The gain is adjusted to measure low persistent light output. Using this adjustment, flash will deflect the spot off the screen so that persistence of the phosphorescent light and. ash cannot be measured at the same time. With proper gain adjustment andv properV load selection and traces on the oscilloscope screen will appear as a series of unconnected vertical lines as shown in Fig. 3. As indicated in the gure' B-I is measured by the vertical interval between thezero line on the oscilloscope screen and the lower end of the rst light trace. B5 would be measured by the interval between the zero line and the lower end of trace number 5. In making persistence measurements, when B-'I is in a readable area, B5 may be oi the screen. But both B-5 and B-l may be read at the same time if the load selector` switch 24| is changed to the next lower position just after B-I is clearly recorded on the screen.

If a high gain photomultiplier tube is used the oscilloscope can be calibrated for milli-foot 1amberts on the 2 megohm position, see 63, Fig. 2. Spot deflection on the oscilloscope is always proportional to the voltage applied to the vertical plates (46B in Fig. 2). Therefore, when a pure resistance load is used for deflection plate bias, deflection will be proportional to the product of load current in the photomultiplier and the resistance load. Therefore, deection is proportional to voltage and V=IARL where IA is the load current and RL is the resistance load. If a capacitive loadv is used, however, the voltage causing deflection becomes equal to where t is the charging time for the capacitance C. For equal spot deection on respective loads then The values of RL and C will always be known'ior any particular measurement since they are read directly from the instrument. However, the charging time must be determined. Taking into consideration the response of the human eye which tends to average out the light intensity rather than record the peak, the charging time is assumed to be .1 second.

' The equivalent resistance RL of a 2ufd condenser would then be t 1 RL--zxio-H If the unit is calibrated in milli-foot lamberts on a 2 megohm resistance, all readings made on the 2 ufd position must be multiplied by o1' 40 to obtain flash measurements in milli-oot lamberts. Similar calculations can be made for other capacitances, other calibrating positions or both.

Using the focusing control 233 and the intensity control 244 on the oscilloscope panel, the spot size'and intensity may beacljusted for most accurate reading. The zero control 205 on the multivibrator panel biases the plates of the oscilloscope so that the deflections are started from a zero base line. The leakage control 225 compensates for inherent leakages in the photornul-A .05 megohrns tiplier.

For flash measurements, the load switch 24!l is merely set on a capacitive load which stores up the total light energy from the screen during the second. The screen undergoes the same excitation as in the persistence measurements. trace on the screen in this case isa series of rounded steps as shown in Fig. 4. The reading is recorded in the same units as B-l,v noting the value of the capacitive load used. Calculations can be made later to convert this reading to millifoot lamberts.

In order to get a purely numerical value for thc ratio F/B-i, the oscilloscope readings may be converted to absolute nulli-foot lamberts as follows.

Amore specific example of an operating circuit embodying apparatus whicli maybe employed in the invention will now be described in detail.

As shown in Fig. 2, a synchronous motor i closes the contacts of a microswitch 2 for a brief time interval, such as 1% second during each revolution,y allowing g 60 cycle synchronizing pulse groups to pass only while the contacts are closed.

The pulse groups are fed to amultivibrator over coupling condenser 3. The multivibrator and 39 are used to adjust the bias at point B to several volts negative. This plus the voltage developed across the cathode resistor I6 presentsa sneden ing time of condenser connected between the.

plate and grid of tube units 8 and 119 respectively. This control resistor also has a xed portion 1.

The multivibrator supplies controlled voltage pulses after amplification by tube l2 to thegrid 3i of the tube 'E5 under test. The repetition time of these pulses is set by adjusting the variable resistor 65 which. together with grid resistor ill, is connected to the grid of tube 4B.

In order to have each pulse of proper time duration or'width, the Width :control is adjusted by means of width control s which is a potentiometer hooked up in the plate circuit oi tube unit 53. The pulseoutput voltage is adjusted by potentiometer and may be read on the D. C. voltmeter i9 in the plate circuit of the pulse amplifying tube |2. The tube l2 which is a buier Vamplifier serves to change the negative pulses of the multivibrator to positive pulses and also to clip the ends of the pulses so that a square voltage wave is obtained. The plate circuit of tube unit #lo is coupled to the grids of bulTer tubes l2 and i9 through condenser |3.

The output of the multivibrator is also connected to'a, parallel buier amplier tube I9, having plate load resistor which may be one unit of a GSNi-GT, and produces positive pulses and also greatly ampliles them for feeding into the pulse 'counter circuit over the series condenser 2G. This condenser divides the pulse voltage between itself and condenser 25. The'ampliiied positive pulses supplied to the counter circuit-have an average magnitude of about 400 volts.

The counter circuit consists mainly of two GHG diodes 2| and 22 connected in inverted relationship and Ya GAG? pentode 3|. During the positive pulse time the condenser 25 is charged in series with condenser 2d over diode 22. Condenser 26 is then discharged during the negative part of the cycle over the inverted diode 2 l. Diode 2| is connected to ground through by-pass condenser 26. Each time a positive pulse occurs ccndenser 25 is given a fractional increase of charge. For practical considerations, since about 200 volts are required to deect the spot i'ive inches on the screen of oscilloscope 4b the total iinal charge on condenser 25 is taken at about 175 volts. When siX pulse steps are desired, the condenser is then charged 3l) v. at each step. The ratio of condensers 2|) and 25 is selected to divide the pulse voltage from tube l@ down to the approximate step voltage. The potentiometer 2l acts as a control to set the exact number of steps.

In the off position, relay 35 is energized, closing contacts l and |532, with |23 connecting the grid of the @Adv tube over the push button contacts a, b to cathode, thus maintaining current flow in the GAG? plate circuit. The multivibrator (8 and di!) is stopped by a -150 v. bias at points B as contact m2 is closed.

When push button 3l is rst depressed, condenser 25 is connected and instantly charged to 150 v. over contacts b, c and H23. Relay 3 snaps then to the pulsing condition, as 150 v is also applied to the grid of tube 3l across current limiting resistor 3d, for which the contacts are set as in Fig. 2. The condenser 25 discharges now over diodes 2| and 22 in series to a negativev value determined by the potentiometer 2, as the relay contact |03 is now open. Thus the potentiometer acts as a step number control by varying the amount of charge left on condenser 25. Release of 3l' applies +75 v. from 33 over contacts a, b and |'0I to resistor 34, thus starting the multivibrator (8 and 40) to furnishpulses to tube I9.

The voltage on Vcondenser increases thus in steps until the negative bias on tube 3| (provided by 75 v. glow tube 32 and adjustable negative bias from potentiometer 2'!) is exceeded, causing tube 3| to energize relay 35 through its plate current. Cathode bias is also supplied to tube 3| through glow tube 33 while 34 is a current limiting resistor to protect glow tube 32'. Energizing relay 35 causes contact |23 to close which closing connects the control grid of tube 3| to the cathode. This causes relay 35 to hold until contact a is opened by again depressing the starting switch. The multivibrator is blocked also since contact |92 is closed. The closing of contact |32 increases Y the bias of tube by 150 v.

The pulse groupA is thus started by depressing starting switch s?, breaking contact a. The plate current continues to flow at first because of the charge on condenser 25 and the multivibrator re- 35V diodes 2| and 22 tothe exact voltage set on po- Y denser-for tube 75.

tentiometer 2'1 which controls the number of charging steps'and hence the number of pulses.

The multivibrator remains blocked until release of the starting switch closes contacta, applying v. over contact |0| through the bleeder resistor 39 for the multivibrator bias and returning point B to operating potential. The cycle then repeats as described. The grid bias of the cathode ray tube under test is controlled by the variable resistor 'H which is connected to grid 8|' through resistor 16. 14 serves as a coupling con- The light flashes on the screen of cathode ray tube 'l5 are picked up by the No. 931 photomultiplier tube i8.' The power supply of the photomultiplier tube (not shown) has a switch 82 for varying the voltage in 150 v. steps.` A Vernier control may be used to Vary the voltage between the v. steps.

The gain of the photomultiplier tube is varied through switch 6'? the highest gain being with the 50 megohm load and the lowest with the 80,000 ohm load. The load resistances 60, 62 and 63 are bypassed with condensers, numbered respectively 59, 6|, and 64 so as to keep a clean spot with high gains on the oscilloscope 46. The smaller load resistors E5 and 66 do not need to be bypassed. Also on the load switchV t? are positions for obtaining various capacitances, numbered 68, 89, ll, and l2, for integrating the light pulses when a Gr. E. recording meter is used. To compensate for leakage to ground on stray light signal, a positive voltage is taken from potentiometer 58, marked compensation, and fedv over resistor 57 to buck out the leakage current.

The readings from which build-up and flashratio are calculated taken from. either a iong persistence oscilloscope such as a CP'I, et, or a G. E. recording meter ilif Wherewit is desired to use both ins uments together, centering voitage from potentiometer $5, connected to ground.

line on the oscilloscope. When the G; E. meter is not used a 1800 ohm resistance is substituted for it.

The horizontal deiiection plate 4ta of the oscilloscope is coupled to the step voltage on 25 by acathode drive D. C. amplifier tube e2. The plate cf tube 12 is connected to a +300 v. power supply while the cathode is connected to a ---150 v. power supply through cathode coupling resistor 43. The cathode of tube 42 is connected to ground over damping resistor 4 and bypass condenser 45. The resistor aids in obtaining a clean spot onthe oscillo'graph at low deflection voltages while 44 and 45 together act as a filter for high frequencies. The vertical deflection plates 46B are plate and cathode coupled by tube 5l. Potentiometer 4! is for setting the zero level on the vertical oscillograph deflection by adjusting the grid bias on tube 5i. The cathode of tube 5I is connected to the i550 v. power supply through cathode resistor 5i) while the plate is connected to the +300 v. power supply through resistors 52 and 53. D. C. coupling is achieved through a 150V. glow tube 41, bypassed by condenser 48, and acting as a negative bias battery. The glow tube requires a small bleeder current through resistor V49.

Readings are taken on the oscilloscope as described previously with the aid oi? block diagram, Fig. 1. which is a long persistence cathode ray tube, of known calibration, is utilized.

I claim as my invention:

For calibration purposes a 5FP'?,

ing the signals corresponding to said light output Yat any lgiven time.

5. A method of measuring the light output characteristics or a luminescent screen which method comprises reducing the screen to a zero energy level, supplying to said screen a series of electron excitations having predetermined inl. In an apparatus for measuring the light I output characteristics of a luminescent screen, means for reducing said screen to a predetermined energy level, means for applying to said screen light producing excitations of predetermined intensity, duration, number, and spacing, means for detecting the light output due to said excitations, and means for recording the signals generated by said light output detecting means.

2. In apparatus for measuring the light output characteristics of a cathode ray tube screen, a multivibrator for supplying successive cycles of standard electron excitations to said screen, a pulse counter circuit for limiting the number of excitations per cycle, a photomultiplier tube for picking up the light output on said screen due to said electron excitations and converting them into a corresponding electron current, and means associated with said photomultiplier tube for recording fluctuations in said electron current.

3. In apparatus for measuring the light output characteristics of a cathode ray tube screen, a multivibrator for supplying successive cycles of standard electron excitations to said screen, a pulse counter circuit for limiting the number of excitations per cycle, a photomultiplier tube for picking up the light output on said screen due to said electron excitations and converting them into a corresponding electron current, and a cathode ray oscilloscope associated with said photomultiplier tube for recording fluctuations in said electron current.

4. A method of measuring the light output characteristics of a luminescent screen which method comprises reducing the screen to a basic energy level, supplying to said screen a series tensity, duration, number, and spacing, detecting the light output due to said excitations and recording the signals corresponding to said light output on a calibrated scale.

5. A method of measuring the persistence light output of a cascade type luminescent screen which method comprises reducing the screen to Zero energy level, supplying to said screen a series oi' excitations having predetermined intensity, duration, number, and spacing, detecting the iight output due to said excitations and recording the signals corresponding to said light output after the first excitation and after the nal excitation of'said series.

7. A method of measuring the ash ratio of a cascade type luminescent screen which method comprises reducing the screen to a basic energy level, supplying to saidy screen a'series of electron excitations having predetermined intensity, duration, number, and spacing, connecting a capacitive load into a photoelectric detecting circuit for storing up the energy detected during each excitation, detecting the total light output dueto said excitations and recording the signals corresponding to` said light output.

8. A method of rating the persistence light output of a cascade type luminescent screen which method comprises reducing the screen to a basic energy level, supplying to said screen a series of standard electron excitations, detecting the light output due to said excitations, recording the signals corresponding to said light output at any given time and comparing the recorded signals with those obtained on a standardized recorder of a type similar to the rst mentioned recorder.

9. A method of measuring the light output characteristics of a luminescent screen which method comprises reducing the screen to a basic energy level, supplying to said screen a series of standard electron excitations, detecting the light output due to said excitations, and recording the signals corresponding to said light output on the calibrated screen of a cathode ray oscilloscope.

10. In an apparatus for measuring the light output characteristics of a luminescent screen, a multivibrator for supplying successive cycles of standard electron excitations to said screen, a phase inverting pulse amplier associated with said multivibrator and a pulse counter circuit connected to the output circuit of said amplier, said pulse counter circuit limiting the number of excitations supplied to the screen during each cycle of the multivibrator, photoelectric means for detecting the light output of said screen due to said excitation and for converting said detected output into an electron current, and means actuated by said electron current for recording the value thereof.

11. In apparatus for measuring the persistent light output characteristics of a phosphor coated screen, means for applying to said screen successive cycles of electron excitations having a predetermined intensity, means for varying the number of said excitations per cycle, means responsive to the light output of said screen due to said excitations for converting said excitations into a aww-1 corresponding electron current, and means responsive to said electron current for recording the fluctuations of said current.

12. In apparatus according to claim l1, means 13. In an apparatus for measuring the persistent light output of a phosphor coated screen, means for applying to said screen a series of electron excitations, means for Varying the spacing of the excitations in said series, means for detecting the light output of said screen due to said. excitations and means synchronized with said spacing means for recording signals generated by said light output detecting means.

14. In an apparatus for measuring the persistent light output of a phosphor coated screen, means for applying to said screen a series of electron excitations, means for varying .the spacing of the excitations in said series, means for detecting the light output of said screen due to said excitations and means synchronized with said spacing means for recording the signals generated by said light output detecting means in response to each of said excitations separately.

15. An apparatus according to Yclaim 13 including means for actuating said recording means at any desired time interval after the occurrence of each of said excitations.

15. In an apparatus for measuring the persistent light output of a phosphor coated screen, means for applying to said screen a series of elec- 12 tron excitations, means for spacing said excitations, means for detecting the light output of said screens due to said excitations and means synchronized with said spacing means for recording signals generated by said light output detecting 17. YIn vapparatus for measuring the persistent light output characteristics of a luminescent screen, means for supplying electron excitations to'said screen, a multivibrator connected to said supplying means for controlling the spacing and intensity of said excitations, means for detecting the light output on saidl screen due to said excitations and means for recording the signals generated by said` light output detecting meansA in response to the light routput oi said. screen.

OTTO HEINRICH SCHADE.

REFERENCES CITED The following references are o1" record in the le of this patent-z Y UNITED STATES PATENTS Number OTHER REFERENCES Zworykin, An Automatic` Recording Spectroradiometer for Cathodoluminescent Materials, J. O. S. A., Feb. 1939, pp. 84 to 91. 

